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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: An XML API using Java streams
 Let me elaborate the thought that new approaches to navigation might profit from a thorough appreciation of the achievements of XPath. One of the profoundest, yet most elusive, aspects of powerful ideas is the basic *units* which they presuppose or create. Object orientation is a wonderful example, as is Functional Programming, and a negative example is the relational database system (problems caused by the lack of a unit matching domain entities, which cannot be captured without ephemeral joins). Over abstraction leads to base entitites which are too shapeless - while in principle you might reassemble them into meaningful constructs, this reassembly is overly complex. Under abstraction leads into a clutter. Concerning navigation, the DOM API is under abstraction. Over abstraction would be the conclusion that, given that navigation is just a mapping of locations to locations, navigation is already implemented by the flatmap() function. Better, but still over abstraction, would be the postulation of two primitives, mapping and filtering (directly supported by the Streams API functions flatmap() and filter().) The achievement of XPath may be described as an ingenious definition of *units* giving structure to the overall operation of navigation. The "navigation is a sequence of steps" view is extremely important, but also trivial. Non-trivial, however, the model of a step, a compelling shape composed of "mapper" and "filter" subunits: step ::= mapper filter* mapper ::= standardMapper | customMapper customMapper ::= expression filter ::= expression standardMapper ::= axisMapper axisFilter axisMapper ::= child | descendant | ancestor | ... axisFilter ::= nameTest | kindTest Our two primitives - mappers and filters - are arranged into three basic units: axisMapper, axisFilter, expression. Now we can decompose navigation into two levels: level#1 steps, level#2 the step components (axis, node test and predicates). The reduction of complexity is so successful, as the technical model matches our mental model of navigation closely. Concerning the Streams API - it is a highly encouraging fact that the key abstractions of navigation - mapping, filtering, chaining - are key abstractions of the API itself (flatmap(), filter(), stream-in-stream-out model). But here come the crossroads: will the emerging navigation API follow XPath in the recognition of a unit "step" which is *ABOVE* the primitives mapper and filter? Looking again at your explorative example: N.walk(Axis.child("city")).map(Axis.attValue("name")) a step model is not yet quite obvious. Interesting, here, also your concern with syntax, which reveals to the eye step units otherwise obfuscated by syntactical clutter. Michael Kay <mike@saxonica.com> schrieb am 17:25 Freitag, 23.Juni 2017:
Well there are certainly some improvements that one could contemplate: * Separating positional filters from boolean filters so they are syntactically distinct * A clearer separation between the "/" and "!" operators * A more orthogonal set of axes, e.g. allowing an "or-self" modifier with any base axis * Representing attributes using maps rather than as nodes But yes, the basic model is pretty sound. Axes are functions from nodes to sequences of nodes, the "!" operator is a flatMap() operator, "/" is flatMap followed by deduplication and sorting-into-document-order, boolean predicates a pure filter() operation. A lot of the messier things come from lack of orthogonality and unnecessary complexity in the data model (notoriously namespaces) and XPath does a pretty good job at smoothing over some terrible cracks. Michael Kay Saxonica
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